Re: Help ZFS FreeBSD 8.0 RC2 Write performance issue

From: Ivan Voras <ivoras_at_freebsd.org>
Date: Wed, 11 Nov 2009 22:52:41 +0100
2009/11/11 Sam Fourman Jr. <sfourman_at_gmail.com>:
> On Wed, Nov 11, 2009 at 2:49 PM, Dan Nelson <dnelson_at_allantgroup.com> wrote:
>> In the last episode (Nov 11), Ivan Voras said:
>>> Sam Fourman Jr. wrote:
>>> > I am running FreeBSD 8.0RC2 and I dont understand why my ZFS/NFS is
>>> > acting weird on writes.  I get ~150mbit writes idk if this is good or
>>> > not?  but it paused for a few seconds every once and awhile.
>>>
>>> You didn't give any "iostat" statistics - I suspect that if you
>>> correlate ifstat and iostat output that you will see that network
>>> "pauses" happen during spikes in IO. You should check for this and post
>>> your results.
>>
>> Yes, iostat would be useful here.  "iostat -zxC 2" will give you per-disk
>> stats plus CPU usage every 2 seconds (CPU may be a factor if you have
>> compression enabled).
>>
>> On a Solaris box I admin, setting zfs_write_limit_override helped stuttering
>> while doing heavy writes.  It's not exported on FreeBSD, but it should be
>> easy to add it as a RW sysctl; it lives in dsl_pool.c and can be tweaked at
>> runtime.  Start big and tune it down so each write burst takes under a
>> second; it looks like you're writing solid for around 6-8 seconds now.  The
>> number will vary depending on your disk speed and how much ARC you have.
>
>
> here are some iostats for you. I do not believe I have compression enabled
> am I mistaken? isn'y SATA2 300MB/s? and I am doing ~6MB/s per disk?
> I built this machine with 4GB of memory because I thought ZFS would like it.
> now maybe a re(4) interface isnt the best choice. if that is the
> problem here I can change it.

Your io/ifstat data is very long but you didn't say if you observed
hiccups in network performance during heavy IO times? Since you don't
have timestamps in your data you are the only one who can say.

> We spent ~$800 on a hardware RAID card thinking that it would help performance
>
> Why is it that with sftp we do not see the pauses in Network transfer?

I think NFS uses sync disk IO access by default, this may be your
problem if you are write-heavy. Try setting vfs.nfsrv.async to 1 to
see if this is the cause of your problems.
Received on Wed Nov 11 2009 - 20:53:03 UTC

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